Feline leukemia virus, subgroup C (FeLV-C) induces pure red cell aplasia (PRCA) in viremic cats. There is an absence of erythroid precursors, reticulocytopenia, and severe anemia. Although all cells are infected, only erythropoiesis is affected. Previous studies demonstrate that the envelope protein of FeLV-C acts as a dominant negative protein in infected cells to interfere with the expression, and thus function, of its cell surface receptor, FLVCR. This led to the hypothesis that FLVCR is critical for CFU-E/proerythoblast differentiation or survival. In the past grant cycle, we cloned the cDNA for feFLVCR, then huFLVCR. The predicted 555 aa, 12-membrane spanning domain protein is a member of the major facilitator superfamily (MFS) of transporter proteins and well conserved in bacteria and C. elegans. In further studies, we established that FLVCR exports cytoplasmic heme (Cell 118:757-66, 2004) and thus provided the first description of a mammalian heme transporter. Although heme is synthesized by all nucleated cells and is critical for aerobic metabolism, free heme is toxic, promoting lipid peroxidation, membrane damage and cell death. This necessitates a tight regulation of heme synthesis, its use in hemoproteins, and its degradation. The goal of this competitive renewal application is to study FLVCR physiology in K562 cells, primary (CD34+) hematopoietic cells, and Flvcr-/- mouse models. Experiments will characterize FLVCR substrate specificity and tissue localization and determine if isoforms are present. We will confirm our hypothesis that FLVCR functions as a safety valve to prevent heme toxicity in erythroid precursors at the time during maturation when heme synthesis is initially upregulated, with direct experimentation. We will also test our hypotheses that FLVCR has a similar function in other tissues (e.g., liver and hematopoietic stem cells) and that its expression on macrophages is important for heme recycling and systemic iron homeostasis. We anticipate that these studies will provide novel insights into a unique cell protection mechanism.